1. Field of the Invention
The present invention relates to an apparatus configured to move a scanning unit to execute scanning and a method for controlling the apparatus. In particular, the present invention relates to an apparatus that employs a direct current (DC) motor as a driving source and a method for controlling the apparatus.
2. Description of the Related Art
A conventional image reading apparatus (image scanner) scans a document by using a scanner unit having a reading sensor while moving the scanner unit from an initial position of the scanner unit (a reference position) to read an image of the document. An image reading apparatus like this generally uses a position detection sensor as a unit for determining an initial position of the scanner unit (i.e., a home position sensor). A direction, in which a scanner unit is moved, is generally called a “main scanning direction” while a direction perpendicular to the main scanning direction is generally called a “sub scanning direction”. In the sub scanning direction, a plurality of image reading sensors (e.g., a charge-coupled device (CCD) sensor) is generally provided.
As a control method for determining an initial position of a scanner unit by using a home position sensor, Japanese Patent Application Laid-Open No. 08-22087 discusses a method that uses a plurality of home position sensors provided in the sub scanning direction. When the method discussed in Japanese Patent Application Laid-Open No. 08-22087 is used in reading divided images, when the scanner unit reaches a position stored by the plurality of home position sensors, the scanner unit can return to a home position that is currently the nearest of the home positions that the scanner unit has gone over.
Furthermore, another conventional image reading apparatuses employ a DC motor as a driving source of the scanner unit.
As a method for detecting variation in the speed of moving the scanner unit and changes in the control to be executed thereafter, Japanese Patent Application Laid-Open No. 10-42584 discusses an image input and output apparatus configured, if the speed of the driving motor of the scanner unit is abnormal, to immediately stop the motor. More specifically, the image input and output apparatus discussed in Japanese Patent Application Laid-Open No. 10-42584 estimates the current speed range based on the history of variation of the speed of the motor. If it is determined that the current speed of the motor has gone out of the estimated speed range, the method discussed in Japanese Patent Application Laid-Open No. 10-42584 determines that the motor is not normally controlled and compulsorily stops the motor.
If the image reading apparatus described above does not have a sensor for detecting the position of the scanner unit, the image reading apparatus cannot use the sensor to determine the initial position of the scanner unit.
The conventional image reading apparatus described above initializes the position of the scanner unit by causing the scanner unit to contact the inner peripheral surface of the housing of the image input and output apparatus itself. After the position of the scanner unit is initialized in the above-described manner, the conventional image reading apparatus determines the initial (home) position by moving the scanner unit by a predetermined distance. The operation for initializing the position of the scanner unit is executed at the time of powering on the image reading apparatus, for example.
The scanner unit position initialization operation described above is executed by controlling the moving speed of the DC motor (the scanner unit driving source) to a low speed by limiting the value of the output current. Accordingly, it takes a long time to initialize the position of the scanner unit. In order to reduce the time taken for initializing the position of the scanner unit, it is necessary to increase the limit value of the output current to drive the scanner unit at a high speed.
However, even when the same limit value of the output current is set to a plurality of apparatuses, the driving torque transmitted to the DC motor during the operation for contacting the scanner unit may become high due to differences in the configuration of the apparatuses. If the driving torque becomes high, the DC motor that drives the scanner unit may continue rotating after the scanner unit actually contacts the inner peripheral surface of the housing of the image reading apparatus. As a result, an encoder, which is mounted on a rotational axis of the DC motor, may also rotate. Accordingly, in this case, the state of the scanner unit contacting the inner peripheral surface of the housing of the image reading apparatus cannot be detected.
Due to the above-described detection failure, the image reading apparatus cannot correctly determine the initial position of the scanner unit. The above-described problem may also arise in an apparatus other than the image reading apparatus configured to move the scanner unit.